1. Field of the Invention
This invention relates to ink jet printing, and more particularly to a thermal ink jet printhead apparatus and method for elimination of misdirected satellite drops by control of the effective meniscus tilt angle of ink at the nozzles of an ink jet printhead.
2. Description of the Prior Art
In existing thermal ink jet printing, the printhead comprises one or more ink filled channels, such as disclosed in U.S. Pat. No. 4,463,359 to Ayata et al., communicating with a relatively small ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. A thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper.
The printhead of U.S. Pat. No. 4,463,359 has one or more inkfilled channels which are replenished by capillary action. A meniscus is formed at each nozzle to prevent ink from weeping therefrom. A resistor or heater is located in each channel upstream from the nozzles. Current pulses representative of data signals are applied to the resistors to momentarily vaporize the ink in contact therewith and form a bubble for each current pulse. Ink droplets are expelled from each nozzle by the growth and collapse of the bubbles. Current pulses are shaped to prevent the meniscus from breaking up and receding too far into the channels, after each droplet is expelled. Various embodiments of linear arrays of thermal ink jet devices are shown such as those having staggered linear arrays attached to the top and bottom of a heat sinking substrate and those having different colored inks for multiple colored printing.
U.S. Pat. No. 4,601,777 to Hawkins et al. discloses several fabricating processes for ink jet printheads, each printhead being composed of two parts aligned and bonded together. One part is substantially a flat heater plate substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes, and the second part is a channel plate substrate having at least one recess anisotropically etched therein to serve as an ink supply manifold when the two parts are bonded together. A linear array of parallel grooves are formed in the second part, so that one end of the grooves communicate with the manifold recess and the other ends are open for use as ink droplet expelling nozzles. Many printheads can be simultaneously made by producing a plurality of sets of heating element arrays with their addressing electrodes on, for example, a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds are produced in a second silicon wafer and, in one embodiment, alignment openings are etched thereon at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks and then bonded together and diced into many separate printheads. A number of printheads can be fixedly mounted on a pagewidth configuration which confronts a moving recording medium for pagewidth printing or individual printheads may be adapted for carriage type ink jet printing. In this patent, the parallel grooves which are to function as the ink channels when assembled are individually milled as disclosed in FIG. 6B or anisotropically etched concurrently with the manifold recess. In this latter fabrication approach, the grooves must be opened to the manifold; either they must be diced open as shown in FIGS. 7 and 8, or an additional isotropic etching step must be included. This invention eliminates the fabrication step of opening the elongated grooves to the manifold when they are produced by etching.
U.S. Pat. No. 4,639,748 to Drake et al. discloses an ink jet printhead similar to that described in the patent to Hawkins et al., but additionally containing an internal integrated filtering system and fabricating process therefor. Each printhead is composed of two parts aligned and bonded together. One part is a substantially flat substrate which contains on the surface thereof a linear array of heating elements and addressing electrodes. The other part is a flat substrate having a set of concurrently etched recesses in one surface. The set of recesses include a parallel array of elongated recesses for use as capillary filled ink channels having ink droplet emitting nozzles at one end and having interconnection with a common ink supplying manifold recess at the other ends. The manifold recess contains an internal closed wall defining a chamber with an ink fill hole. Small passageways are formed in the internal chamber walls to permit passage of ink therefrom into the manifold. Each of the passageways have smaller cross-sectional flow areas than the nozzles to filter the ink, while the total cross sectional flow area of the passageways is larger than the total cross sectional flow area of the nozzles. As in Hawkins et al., many printheads can be simultaneously made by producing a plurality of sets of heating element arrays with their addressing electrodes on a silicon wafer and by placing alignment marks thereon at predetermined locations. A corresponding plurality of sets of channels and associated manifolds with internal filters are produced on a second silicon wafer and in one embodiment alignment openings are etched thereon at predetermined locations. The two wafers are aligned via the alignment openings and alignment marks, then bonded together and diced into many separate printheads.
Misdirected satellite drops can be produced by conventional thermal ink jet printheads and can result in observable print quality defects. Such misdirected satellite drops are typically generated when the plane of the ink meniscus in the channel deviates by more than a certain amount from perpendicular to the plane of the channels.